The present invention relates generally to the production of ethanol in a fermentation process. More particularly, the invention relates to the production of fermented beverages such as beers and wines.
As utilized hereinafter, the term "beer" means the potable product of fermentation of brewers wort by appropriate yeasts, and includes specific beverages such as ale, lager, stout and porter. The term "alcoholic malt beverages" also is employed herein as synonymous with beer.
The term "wine" as employed herein means an alcoholic beverage made by yeast fermentation of a must derived from fruits or berries, particularly grapes.
In the description which follows, the production of beer is primarily discussed so as to more easily and readily exemplify and illustrate the essential features of the invention and the differences between the invention and the prior art. As will be appreciated by those of skill in the art, the features discussed also have applicability to wine production, subject to modifications apparent to skilled art workers and/or as pointed out hereinafter, and general applicability to the production of ethanol per se.
Although many nuances exist with respect to particular processing conditions, equipment, yeast strain and the like which differ from one beer producer to the next, the essential feature of all beer production processes is the bringing together of yeast and brewers wort under conditions whereby carbohydrates (sugars) in the wort are converted to alcohol, with evolution of carbon dioxide. For conventional batch processes, this so-called "primary" fermentation step is conducted until a desired degree of attenuation has been achieved, i.e., when fermentation has proceeded to the point where the appropriate degree of sugar conversion to alcohol has occurred and the yeast has consumed a desired quantity of amino acids and other nutrients. Thereafter, the fermented product is matured in processes variously referred to as conditioning, lagering or ruh storage to develop and/or eliminate various aromas and flavors.
The overall time required for beer production is extremely lengthy, sometimes requiring months but more typically requiring from two to three weeks from primary fermentation to final maturation. As a consequence, to achieve the production rates which are demanded by the increasing popularity of beer, large fermentation batches, large and expensive equipment and a number of production lines are required. These features, of course, add significantly to the cost of producing beer.
Another factor which influences the size of fermentation vessels, the overall capacity of beer production processes and the economics of beer production is the problem of foam formation during fermentation. During the course of fermentation, a fine white head appears near the sides of the vessel and above any attemperating coils about 8 to 16 hours after pitching. As fermentation continues, rocky or cauliflower heads of foam appear ("krausen") and eventually reach maximum development ("high krausen"). Depending upon the type, shape and size of the fermentation vessel and the speed of fermentation, the total foam volume can become as much as one-third of the total fermentation volume. At the end of primary fermentation (e.g., 5 to 7 days), the head gradually collapses leaving a dark colored, bitter tasting scum which must be separated from the beer by skimming or suction. While methods are known for minimizing foam formation and facilitating its removal, these too add to the cost and complexity of beer production.
Workers in the art have proposed a number of techniques for achieving more rapid and/or more efficient production of beer, particularly with respect to accelerating the primary fermentation process. For example, it is well known that if the temperature during fermentation (either top or bottom fermentation) is increased, the rate of fermentation can be increased and the fermentation time shortened considerably. It also is known that vigorous exogeneous agitation (i.e., agitation above that naturally occurring by virtue of the evolution of carbon dioxide by the fermenting yeast) can accelerate the rate of fermentation. However, equally well known is the fact that beers produced according to these methods have an undesirable "winey" off-flavor which has been related to increased amounts of volatile compounds, such as higher alcohols and esters. In addition, these techniques also promote excessive yeast growth. See, for example, U.S. Pat. No. 4,068,005 of Chicoye, et al., issued Jan. 10, 1978, and U.S. Pat. No. 3,437,490 of Schaus, et al., issued Apr. 8, 1969.
Another approach to reducing the time required to produce beer is to conduct the operation on a continuous basis. According to one proposed form of continuous operation, a number of vessels is employed for the fermentation, each containing a constant volume of wort and yeast in a particular state of fermentation, fresh wort being continuously added at one end of the vessel train and wholly of partly fermented wort being continuously removed from a vessel at the other end of the vessel train. Beers produced according to such methods have not achieved satisfactory flavor, and the process involves complicated equipment and undue risk of contamination as a consequence of the numerous material transfers required and the typically open nature of the vessels.
There also has been proposed a continuous wort fermentation process wherein yeast are immobilized in calcium alginate gels which are then packed in a reactor through which wort is continuously passed. See, S. E. Godtfredsen, et al., "Application Of Immobilized Yeast And Yeast Coimmobilized With Amyloglucosidase In The Brewing Process", EBC Congress, pp. 505-511 (1981). The authors demonstrate, for a small-scale system, a significant increase in output per unit volume fermentation in this process.
As these authors also point out, however, the very speed with which fermentation is conducted in this continuous process can be self-defeating, a problem which also plagues the earlier-described methods for increasing fermentation rates by means of exogenous agitation and/or increased temperature. Thus, while all these methods may result in an increase in the rate at which sugars in the wort are converted to alcohol, they also limit the amount of time during which yeast, in the process of effecting sugar or carbohydrate conversion, performs other beneficial functions. This is particularly so with respect to the action of yeast on compounds such as diacetyl which are produced during fermentation. Diacetyl has a distinct buttery flavor which is unacceptable in beers. In conventional fermentation, within the time period in which yeast convert the wort to a desired degree of attenuation, the yeast absorb diacetyl (and convert diacetyl precursors to diacetyl which is then absorbed). As a result, the fermented wort contains desirably low levels of diacetyl, with further reduction of diacetyl and other compounds such as hydrogen sulfide and acetaldehyde--which are primary components of the "green" aroma of beer after primary fermentation--being accomplished during maturation processes.
Techniques for increasing the speed of fermentation, therefore, limit the time during which the yeast can act upon and absorb diacetyl (and/or precursors of diacetyl) and other compounds. The beer obtained from primary fermentation using these methods has an unacceptably high level of these undesired compounds and must either undergo prolonged maturation to effect reduction of the level of these compounds and/or rely upon other means to effect such reduction. See, e.g., Shovers, et al., U.S. Pat. No. 3,733,205 regarding the addition of diacetyl reductase and its co-factor, nicotinamide adenine dinucleotide, to fermented wort to remove diacetyl. In either case, the net overall processing time and/or expense of beer production is not materially improved over that achieved using conventional fermentation techniques.
Apart from the foregoing, continuous processes involving reactors or columns packed with immobilized yeast also present other difficulties. For example, wort must be very clear before entering the reactor in order to prevent clogging of the reactor and flow lines. In addition, since it is necessary for fresh wort to be continuously available for fermentation, a considerable supply of wort must be on hand at all times, requiring suitable chilled holding vessels to reduce the risk of bacterial contamination and suitable heating devices for warming the wort prior to fermentation. Finally, adoption of a continuous process is rarely economical for breweries operating according to conventional batch techniques, since the required major equipment changes typically will outweigh any benefit of reduced processing time.
There exists a need in the art of beer and wine production for a process which is capable of reducing the overall time required to achieve an acceptable product and which can be adopted by existing producers without need for extensive and costly equipment modifications. In addition, of course, measures effective to reduce the time required to produce acceptable beers and wines also will have applicability in all processes where alcohol production via fermentation is involved.